Ethylbenzene conversion and xylene isomerization processes and catalysts therefor

ABSTRACT

Catalysts comprising a combination of molecular sieve having a pore diameter of from about 4 to 8 angstroms and a catalytically-effective amount of molybdenum hydrogenation component in an amorphous aluminum phosphate binder provide processes for isomerizing xylene and dealkylating ethylbenzene in feed streams that exhibit stability, selectivity and low ring loss.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Division of application Ser. No. 11/225,976 filedSep. 14, 2005, now U.S. Pat. No. 7,301,064, the contents of which arehereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

This invention relates to catalysts and catalytic processes for theisomerization of xylenes and for the conversion of ethylbenzene bydealkylation in the presence of hydrogen, particularly such catalystsusing a combination of molecular sieve and molybdenum and their use toenhance isomerization activity while retaining ethylbenzene dealkylationactivity, low transalkylation activity and low aromatic ring loss tonaphthenes.

BACKGROUND OF THE INVENTION

Numerous processes have been proposed for the isomerization of one ormore of xylenes (meta-xylene, ortho-xylene and para-xylene) to formother isomers of xylene. In many instances, the sought xylene isomer ispara-xylene due to the demand for terephthalic acid for the manufactureof polyester.

In general, these xylene isomerization processes comprise contacting thexylene isomer sought to be isomerized with an isomerization catalystunder isomerization conditions. Various catalysts have been proposed forxylene isomerization. These catalysts include molecular sieves,especially molecular sieves contained in a refractory, inorganic oxidematrix. The catalysts also contain a hydrogenation metal component.

Due to the large scale of commercial facilities to produce para-xyleneon an economically competitive basis, not only must a xyleneisomerization process be active and stable, but it also must not undulycrack the aromatic feed so as to result in ring loss. Moreover, theisomerization processes produce by-products such as benzene, toluene,and aromatics having 9 or more carbon atoms. Often the xylene-containingfeed to be isomerized also contains ethylbenzene. Ethylbenzene may bedealkylated, or the ethylbenzene can be converted by isomerization ortransalkylation. Whether the isomerization process will dealkylate orwill convert ethylbenzene depends upon the isomerization conditionsincluding catalyst.

Where the ethylbenzene is sought to be dealkylated, several concernsexist. First, the dealkylation should be selective to the ethylbenzeneand not cause undue loss of xylene. Second, the isomerization andethylbenzene conversion should not result in undue production oftransalkylated products such as toluene and trimethylbenzene. Third, thedealkylation should not cause the production of naphthenes that wouldcontaminate any benzene stream separated from the product of theisomerization and ethylbenzene conversion, and thus reduce the value ofthe benzene.

Catalysts containing molybdenum provide advantageously low production ofnaphthenes. However, they do not exhibit a good balance between xyleneisomerization and ethylbenzene dealkylation. A catalyst exhibiting goodethylbenzene conversion provides a low ratio of para-xylene to totalxylenes. If the concentration of molybdenum is increased, someimprovement can be obtained in xylene isomerization, but at a cost inethylbenzene conversion activity.

Another difficulty with molybdenum-containing catalysts is that thexylene isomerization activity deteriorates with decreasing hydrogenpartial pressure to an undesirable extent. Consequently, such a catalystwould not be useful in xylene isomerization facilities that use lowerpressures.

U.S. Pat. No. 4,362,653, for instance, discloses a hydrocarbonconversion catalyst which could be used in the isomerization ofisomerizable alkylaromatics that comprises silicalite (having anMFI-type structure) and a silica polymorph. The catalyst may containoptional ingredients. Molybdenum is listed as one of the many optionalingredients. U.S. Pat. No. 4,899,012 discloses catalyst forisomerization and conversion of ethylbenzene containing a Group VIIImetal, lead, a pentasil zeolite and an inorganic oxide binder. U.S. Pat.No. 6,573,418 discloses a pressure swing adsorption process to separatepara-xylene and ethylbenzene from C₈ aromatics. Included among thecatalysts disclosed for ethylbenzene isomerization are those containingZSM-5 type of molecular sieve (Al-MFI) dispersed on silica. Thecatalysts contain a hydrogenation metal and listed among thehydrogenation metals are molybdenum. Suitable matrix materials are saidto be alumina and silica. See example 12 which uses amolybdenum-containing catalyst for xylene isomerization.

A catalyst using molybdenum tends to generate less naphthenes than aplatinum-containing catalyst. However, at comparable ethylbenzeneconversions, the molybdenum-containing catalysts have been inferior toplatinum-containing catalysts in isomerization activity, i.e., yields axylene product distribution not as close to equilibrium as are theproducts using a platinum-containing catalyst. Hence,platinum-containing catalysts have been preferred for commercial use.Catalysts are sought that provide the combination of the low naphthenegeneration achievable with molybdenum catalysts with good ethylbenzeneconversion and isomerization activity. Moreover, catalysts are soughtthat can be used in a xylene isomerization facility regardless ofwhether it operates at lower pressures, e.g., about 700 pKa, or higherpressures, e.g., 1500 kPa.

SUMMARY OF THE INVENTION

In accordance with this invention, molybdenum-containing catalysts andprocesses for using the catalysts are provided for the isomerization ofxylene and the dealkylation of ethylbenzene that exhibit not onlydesirable ethylbenzene conversion activity but also desirableisomerization activity, i.e., approach to xylene equilibriumdistribution, while retaining low naphthene generation activity. Hence,the advantages provided by the catalysts of the invention can renderthem viable alternatives to platinum-containing catalysts. Theimprovement in molybdenum-containing catalysts provided by thisinvention resides in subjecting the catalyst to sufficient sulfiding toenhance the xylene isomerization activity of the catalyst.

The catalysts in accordance with this invention have a combination of acatalytically-effective amount of molecular sieve having a pore diameterof from about 4 to 8 angstroms, preferably a pentasil structure zeolite,e.g., MFI-type zeolite, and a catalytically-effective amount ofmolybdenum hydrogenation component which has been subjected tosufficient sulfiding to enhance the xylene isomerization activity of thecatalyst. The sulfiding of the catalyst may be effected during at leastone of (i) the preparation of the catalyst, (ii) regeneration of thecatalyst or (iii) use of the catalyst in an isomerization process. Oftenthe catalyst comprises sulfur in a S:Mo atomic ratio of up to about 3:1,preferably about 0.01:1 to 3:1, and most preferably about 0.1:1 to 2:1.

The broad aspects of the processes of this invention comprise contactinga feed stream containing a non-equilibrium admixture of at least onexylene isomer and ethylbenzene, wherein preferably between about 1 and60, and more frequently between about 5 and 35, mass-% of the feedstream is ethylbenzene, with a catalyst comprising acatalytically-effective amount of the catalysts of this invention toprovide an isomerization product having a reduced ethylbenzeneconcentration and an isomerized xylene distribution. The isomerizationconditions include the presence of hydrogen in a mole ratio tohydrocarbon of between about 0.5:1 to 6:1, preferably 2:1 to 5:1.Preferably, the isomerization is conducted under at least partiallyvapor phase conditions. In the preferred aspects of the processes ofthis invention, the ethylbenzene conversion is at least about 60 mass-%and the xylene distribution is at least 90, preferably at least about95, percent of equilibrium. Often, the naphthene net make is less than0.02 mass-% based on the xylenes and ethylbenzene in the feed.

DETAILED DESCRIPTION OF THE INVENTION

The Catalyst

The catalysts used in the processes of this invention comprise amolecular sieve having a pore diameter of from about 4 to 8 angstroms,and a molybdenum hydrogenation component in an amorphous aluminumphosphate binder. Examples of molecular sieves include those havingSi:Al₂ ratios greater than about 10, and often greater than about 20,such as the MFI, MEL, EUO, FER, MFS, MTT, MTW, TON, MOR, UZM-8 and FAUtypes of zeolites. Pentasil zeolites such as MFI, MEL, MTW and TON arepreferred, and MFI-type zeolites, such as ZSM-5, silicalite, Borolite C,TS-1, TSZ, ZSM-12, SSZ-25, PSH-3, and ITQ-1 are especially preferred.

The zeolite is combined with binder for convenient formation of catalystparticles. The relative proportion of zeolite in the catalyst may rangefrom about 1 to about 99 mass-%, with about 2 to about 90 mass-% beingpreferred.

The binder should be uniform in composition and relatively refractory tothe conditions used in the process. Suitable binders include inorganicoxides such as one or more of alumina, aluminum phosphate, magnesia,zirconia, chromia, titania, boria and silica. The catalyst also maycontain, without so limiting the composite, one or more of (1) otherinorganic oxides including, but not limited to, beryllia, germania,vanadia, tin oxide, zinc oxide, iron oxide and cobalt oxide; (2)non-zeolitic molecular sieves, such as the aluminophosphates of U.S.Pat. No. 4,310,440, the silicoaluminophosphates of U.S. Pat. No.4,440,871 and ELAPSOs of U.S. Pat. No. 4,793,984; and (3) spinels suchas MgAl₂O₄, FeAl₂O₄, ZnAl₂O₄, CaAl₂O₄, and other like compounds havingthe formula MO—Al₂O₃ where M is a metal having a valence of 2; whichcomponents can be added to the composite at any suitable point.

A preferred binder or matrix component comprises an amorphousphosphorous-containing alumina (hereinafter referred to as aluminumphosphate) component. The atomic ratios of aluminum to phosphorus in thealuminum phosphate binder/matrix generally range from about 1:10 to100:1, and more typically from about 1:5 to 20:1. Preferably thealuminum phosphate has a surface area of up to about 450 m²/g, andpreferably the surface area is up to about 250 m²/g. See, for instance,U.S. Pat. No. 6,143,941.

When used, the amount of the aluminum phosphate binder is preferablysufficient to reduce the transalkylation activity of the catalyst, e.g.,co production of toluene and trimethylbenzene. The preferred catalystsof this invention can be characterized as having under specifiedconditions, a net make of toluene and trimethylbenzene of less thanabout 3, preferably less than about 2, mass-% based on the mass of C₈aromatics (xylenes and ethylbenzene) in the feed. The EvaluationConditions for this characterization comprise using feed streamcontaining 15 mass-% ethylbenzene, 25 mass-% ortho-xylene and 60 mass-%meta-xylene; a hydrogen to hydrocarbon ratio of 4:1; a pressure of 700kPa gauge; a weight hourly space velocity of 10 hr⁻¹, and a temperaturesufficient to convert 75 mass-% of the ethylbenzene with the data takenat 50 hours of operation. These specified conditions are for the purposeof providing common conditions for catalyst evaluation and are notlimiting as to the xylene isomerization conditions that may be used inthe processes of this invention.

The aluminum phosphate may be prepared in any suitable manner. Onesuitable technique for preparing aluminum phosphate is the oil-dropmethod of preparing the aluminum phosphate which is described in U.S.Pat. No. 4,629,717. This technique involves the gellation of a hydrosolof alumina which contains a phosphorus compound using the well-knownoil-drop method. Generally this technique involves preparing a hydrosolby digesting aluminum in aqueous hydrochloric acid at refluxtemperatures of about 80° to 105° C. The mass ratio of aluminum tochloride in the sol often ranges from about 0.7:1 to 1.5:1. A phosphoruscompound is added to the sol. Preferred phosphorus compounds arephosphoric acid, phosphorous acid and ammonium phosphate. The relativeamount of phosphorus and aluminum expressed in atomic ratios ranges fromabout 10:1 to 1:100, and often 10:1 to 1:10.

If desired, the molecular sieve can be added to the hydrosol prior togelling the mixture. One method of gelling involves combining a gellingagent with the mixture and then dispersing the resultant combinedmixture into an oil bath or tower which has been heated to elevatedtemperatures such that gellation occurs with the formation of spheroidalparticles. The gelling agents which may be used in this process arehexamethylene tetraamine, urea or mixtures thereof. The gelling agentsrelease ammonia at the elevated temperatures which sets or converts thehydrosol spheres into hydrogel spheres. The spheres are thencontinuously withdrawn from the oil bath and typically subjected tospecific aging and drying treatments in oil and in ammoniacal solutionto further improve their physical characteristics. The resulting agedand gelled particles are then washed and dried at a relatively lowtemperature of about 100° to 150° C. and subjected to a calcinationprocedure at a temperature of about 450° to 700° C. for a period ofabout 1 to 20 hours.

The combined mixture preferably is dispersed into the oil bath in theform of droplets from a nozzle, orifice or rotating disk. Alternatively,the particles may be formed by spray-drying of the mixture at atemperature of from about 425° to 760° C. In any event, conditions andequipment should be selected to obtain small spherical particles; theparticles preferably should have an average diameter of less than about5.0 mm, more preferably from about 0.2 to 3 mm, and optimally from about0.3 to 2 mm.

Alternatively, the catalyst may be an extrudate. The well-knownextrusion method initially involves mixing of the molecular sieve withoptionally the binder and a suitable peptizing agent to form ahomogeneous dough or thick paste having the correct moisture content toallow for the formation of extrudates with acceptable integrity towithstand direct calcination. Extrudability is determined from ananalysis of the moisture content of the dough, with moisture content inthe range of from about 30 to about 50 mass-% being preferred. The doughis then extruded through a die pierced with multiple holes and thespaghetti-shaped extrudate is cut to form particles in accordance withtechniques well known in the art. A multitude of different extrudateshapes is possible, including, but not limited to, cylinders,cloverleaf, dumbbell and symmetrical and asymmetrical polylobates. It isalso within the scope of this invention that the extrudates may befurther shaped to any desired form, such as spheres, by marumerizationor any other means known in the art.

Another alternative is to use a composite structure having a core and anouter layer containing molecular sieve and aluminum phosphate. Often,the thickness of the molecular sieve layer is less than about 250microns, e.g., 20 to 200, microns. The core may be composed of anysuitable support material such as alumina or silica, and is preferablyrelatively inert towards dealkylation. Advantageously, at least about75, and preferably at least about 90, mass-% of the molybdenum in thecatalyst is contained in the outer layer. The catalyst may be in anysuitable configuration including spheres and monolithic structures.

The catalyst may contain other components provided that they do notunduly adversely affect the performance of the finished catalyst. Thesecomponents are preferably in a minor amount, e.g., less than about 40,and most preferably less than about 15, mass-% based upon the mass ofthe catalyst. These components include those that have found applicationin hydrocarbon conversion catalysts such as: (1) refractory inorganicoxides such as alumina, titania, zirconia, chromia, zinc oxide,magnesia, thoria, boria, silica-alumina, silica-magnesia,chromia-alumina, alumina-boria, silica-zirconia, phosphorus-alumina,etc.; (2) ceramics, porcelain, bauxite; (3) silica or silica gel,silicon carbide, clays and silicates including those syntheticallyprepared and naturally occurring, which may or may not be acid treated,for example, attapulgite clay, diatomaceous earth, fuller's earth,kaolin, kieselguhr, etc.; and (4) combinations of materials from one ormore of these groups. Often, no additional binder component need beemployed.

Catalysts of the invention comprise molybdenum as a hydrogenationcatalyst component. If desired, the catalyst may contain, as a minorportion of the hydrogenation catalyst component, a platinum-group metal,including one or more of platinum, palladium, rhodium, ruthenium osmium,and iridium. In any event, the molybdenum comprises at least about 60atomic-percent, preferably at least about 80 atomic-percent toessentially all, of the hydrogenation metal (elemental basis) of thehydrogenation component. Often, any platinum group metal present is inan amount of 20 to 500 parts per million by mass (ppm-mass). Molybdenum(calculated on an elemental basis) generally comprises from about 0.1 toabout 5 mass-% of the final catalyst. The hydrogenation metal may existwithin the final catalyst composite as a compound such as an oxide,sulfide, halide, oxysulfide, etc., or as an elemental metal or incombination with one or more other ingredients of the catalystcomposite.

It is within the scope of the present invention that the catalystcomposites may contain other metal components. Such metal modifiers mayinclude rhenium, tin, germanium, lead, cobalt, nickel, indium, gallium,zinc, uranium, dysprosium, thallium, and mixtures thereof. Catalyticallyeffective amounts of such metal modifiers may be incorporated into thecatalysts by any means known in the art to effect a homogeneous orstratified distribution. The catalysts of the present invention maycontain a halogen component, comprising fluorine, chlorine, bromine oriodine or mixtures thereof, with chlorine being preferred. Preferably,however, the catalyst contains no added halogen other than thatassociated with other catalyst components.

The hydrogenation metal component may be incorporated into the catalystcomposite in any suitable manner. One method of preparing the catalystinvolves the utilization of a water-soluble, decomposable compound ofthe hydrogenation metal to impregnate the calcined sieve/bindercomposite. Alternatively, a hydrogenation metal compound may be added atthe time of compositing the sieve component and binder. One usefulprocess for making the catalysts comprises forming the catalystcomposite without the molybdenum component and then impregnating orotherwise depositing on the composite with a molybdenum compound such asammonium heptamolybdate, molybdenum trioxide, ammonium dimolybdate,molybdenum oxychloride, molybdenum halides, e.g., molybdenum chlorideand molybdenum bromide, molybdenum carbonyl, phosphomolybdates, andheteromolybdic acids. Usually water soluble molybdenum compounds areselected as the source of the molybdenum component for the catalyst.

The catalyst composites are dried at a temperature of from about 100° toabout 320° C. for a period of from about 2 to about 24 or more hours. Ifdesired, the catalyst may be calcined at a temperature of from about400° to about 650° C. in an air atmosphere for a period of from about0.1 to about 10 hours. Steam may also be present during the calcination,e.g., from about 0.5 to 20, say, about 1 to 10, mol-% steam based on theair. Where the catalyst contains a minor amount, based on totalhydrogenation metal, of platinum group metal, the resultant calcinedcomposites often are subjected to a substantially water-free reductionstep to ensure a uniform and finely divided dispersion of the optionalmetallic components. The reducing agent contacts the catalyst atconditions, including a temperature of from about 200° to about 650° C.and for a period of from about 0.5 to about 10 hours, effective toreduce substantially all of the platinum group metal component to themetallic state.

The catalysts are subjected to sulfiding to enhance activity. Sulfidingconditions are those in which the sulfiding agent is incorporated intothe catalyst without forming sulfur dioxide. The sulfiding may be doneduring the catalyst preparation or thereafter, including as apretreatment at catalyst start-up or during use of the catalyst. Thesulfiding may be conducted in any convenient manner. For instance, asolid or sorbed sulfur-containing component, i.e., sulfiding agent, maybe incorporated into the catalyst composite which decomposes during thecatalyst preparation or during start-up or use of the catalyst.Alternatively, the formed catalyst may be contacted with a liquid orgaseous sulfiding agent under sulfiding conditions. Examples ofsulfiding agents include hydrogen sulfide, carbonyl sulfide, carbondisulfide, salts, especially ammonium and organo salts, of sulfates,bisulfates, sulfites, and bisulfites, sulfur dioxide, sulfur trioxide,organosulfides, e.g., dimethyl sulfide, diethyl sulfide, and methylethyl sulfide; mercaptans, e.g., methyl mercaptan, ethyl mercaptan, andt-butyl mercaptan; thiophenes, e.g., tetrahydrothiophene.

The sulfiding conditions can vary widely and will depend upon the natureto the sulfiding agent and the extent of sulfiding desired. Forinstance, with oxygen-containing sulfur compounds, the sulfidingconditions should be sufficient to reduce the sulfur moiety to sulfide.The selection of the sulfiding conditions will also be influenced limitsof feasibility at the location of the catalyst undergoing sulfiding.Thus, different conditions may be preferred where the sulfiding is beingconducted after the catalyst has been installed in a reactor for theisomerization as would be preferred where the catalyst is at a facilityfor the manufacture of catalyst. In general, the sulfiding may beconducted over a temperature range of 0° to 800° C., preferably about10° to 500° C. and a pressure of from about 10 to 5000 or more kPaabsolute. The duration of the sulfiding will depend upon the otherconditions of the sulfiding, e.g., the sulfiding agent, theconcentration of the sulfiding agent, and sulfiding temperature, as wellas the amount of sulfur to be incorporated into the catalyst. Usuallythe sulfiding is conducted for a period of time of at least about 10minutes, and may, in the case of in situ sulfiding in an isomerizationreactor, be continuous. Where the sulfiding is accomplished during thepreparation of the catalyst, the sulfiding is usually done over a periodof at least about 10 minutes, e.g., 10 minutes to 24 hours. Often, thesulfiding is done in the presence of hydrogen, e.g., at a partialpressure of about 10 to 5 MPa.

Where sulfiding is done while the catalyst is in an isomerizationreactor, the sulfiding may be accomplished as a pretreatment or duringthe isomerization process itself. In the latter case, the sulfidingagent is usually provided in a low concentration, e.g., less than about50, say about 0.001 to 20, ppm-mass of the feedstock.

Catalysts may be regenerated. Where the loss of catalytic activity isdue to coking of the catalyst, conventional regeneration processes suchas high temperature oxidation of the carbonaceous material on thecatalyst may be employed. In an aspect of this invention, the catalystsuffering from the loss of isomerization activity may be regenerated bysulfiding to regain at least a portion of the isomerization activity.The regeneration may also reduce the naphthene make of the catalyst.Advantageously, the regeneration can occur while the catalyst is in theisomerization reactor, either by intermittently supplying sulfidingagent during the isomerization process at times that regeneration issought, or by terminating the isomerization process and conducting adedicated sulfiding. When in situ regeneration is desired, the sulfidingagent is usually provided in an amount between about 0.1 and 20 ppm-massbased upon the feed stream, for a time sufficient to introduce betweenabout 0.5 and 3 atoms of sulfur per atom of molybdenum.

The Process

The feedstocks to the aromatics isomerization process of this inventioncomprise non-equilibrium xylene and ethylbenzene. These aromaticcompounds are in a non-equilibrium mixture, i.e., at least one C₈aromatic isomer is present in a concentration that differs substantiallyfrom the equilibrium concentration at isomerization conditions. Thus, anon-equilibrium xylene composition exists where one or two of the xyleneisomers are in less than equilibrium proportion with respect to theother xylene isomer or isomers. The xylene in less than equilibriumproportion may be any of the para-, meta- and ortho-isomers. As thedemand for para- and ortho-xylenes is greater than that for meta-xylene,usually, the feedstocks will contain meta-xylene. Generally the mixturewill have an ethylbenzene content of about 1 to about 60 mass-%, anortho-xylene content of 0 to about 35 mass-%, a meta-xylene content ofabout 20 to about 95 mass-% and a para-xylene content of 0 to about 30mass-%. Usually the non-equilibrium mixture is prepared by removal ofpara-, ortho- and/or meta-xylene from a fresh C₈ aromatic mixtureobtained from an aromatics-production process. The feedstocks maycontain other components, including, but not limited to naphthenes andacyclic paraffins, as well as higher and lower molecular weightaromatics.

The alkylaromatic hydrocarbons may be used in the present invention asfound in appropriate fractions from various refinery petroleum streams,e.g., as individual components or as certain boiling-range fractionsobtained by the selective fractionation and distillation ofcatalytically cracked or reformed hydrocarbons. Concentration of theisomerizable aromatic hydrocarbons is optional; the process of thepresent invention allows the isomerization of alkylaromatic-containingstreams such as catalytic reformats with or without subsequent aromaticsextraction to produce specified xylene isomers and particularly toproduce para-xylene.

According to the process of the present invention, the feedstock, in thepresence of hydrogen, is contacted with the catalyst described above.Contacting may be effected using the catalyst system in a fixed-bedsystem, a moving-bed system, a fluidized-bed system, and anebullated-bed system or in a batch-type operation. In view of the dangerof attrition loss of valuable catalysts and of the simpler operation, itis preferred to use a fixed-bed system. In this system, the feed mixtureis preheated by suitable heating means to the desired reactiontemperature, such as by heat exchange with another stream if necessary,and then passed into an isomerization zone containing catalyst. Theisomerization zone may be one or more separate reactors with suitablemeans therebetween to ensure that the desired isomerization temperatureis maintained at the entrance to each zone. The reactants may becontacted with the catalyst bed in upward-, downward-, or radial-flowfashion.

The isomerization is conducted under isomerization conditions includingisomerization temperatures generally within the range of about 100° toabout 550° C. or more, and preferably in the range from about 150° to500° C. The pressure generally is from about 10 kPa to about 5 MPaabsolute, preferably from about 100 kPa to about 3 MPa absolute. Theisomerization conditions comprise the presence of hydrogen in a hydrogento hydrocarbon mole ratio of between about 0.5:1 to 6:1, preferablyabout 1:1 or 2:1 to 5:1. One of the advantages of the processes of thisinvention is that relatively low partial pressures of hydrogen are stillable to provide the sought selectivity and activity of the isomerizationand ethylbenzene conversion. A sufficient mass of catalyst comprisingthe catalyst (calculated based upon the content of molecular sieve inthe catalyst composite) is contained in the isomerization zone toprovide a weight hourly space velocity with respect to the liquid feedstream (those components that are normally liquid at STP) of from about0.1 to 50 hr⁻¹, and preferably 0.5 to 25 hr⁻¹.

The isomerization conditions may be such that the isomerization isconducted in the liquid, vapor or at least partially vaporous phase. Forconvenience in hydrogen distribution, the isomerization is preferablyconducted in at least partially in the vapor phase. When conducted atleast partially in the vaporous phase, the partial pressure of C₈aromatics in the reaction zone is preferably such that at least about 50mass-% of the C₈ aromatics would be expected to be in the vapor phase.Often the isomerization is conducted with essentially all the C₈aromatics being in the vapor phase.

Usually the isomerization conditions are sufficient that at least about50, preferably between about 60 and 80 or 90, percent of theethylbenzene in the feed stream is converted. Generally theisomerization conditions do not result in a xylene equilibrium beingreached. Where the isomerization process is to generate para-xylene,e.g., from meta-xylene, the feed stream contains less than 5 mass-%para-xylene and the isomerization product comprises a para-xylene toxylenes mole ratio of between about 0.233:1 to 0.25:1 preferably atleast about 0.235:1.

The particular scheme employed to recover an isomerized product from theeffluent of the reactors of the isomerization zone is not deemed to becritical to the instant invention, and any effective recovery schemeknown in the art may be used. Typically, the isomerization product isfractionated to remove light by-products such as alkanes, naphthenes,benzene and toluene, and heavy byproducts to obtain a C₈ isomer product.Heavy byproducts include dimethylethylbenzene and trimethylbenzene. Insome instances, certain product species such as ortho-xylene ordimethylethylbenzene may be recovered from the isomerized product byselective fractionation. The product from isomerization of C₈ aromaticsusually is processed to selectively recover the para-xylene isomer,optionally by crystallization. Selective adsorption is preferred usingcrystalline aluminosilicates according to U.S. Pat. No. 3,201,491.Improvements and alternatives within the preferred adsorption recoveryprocess are described in U.S. Pat. No. 3,626,020, U.S. Pat. No.3,696,107, U.S. Pat. No. 4,039,599, U.S. Pat. No. 4,184,943, U.S. Pat.No. 4,381,419 and U.S. Pat. No. 4,402,832, incorporated herein byreference.

EXAMPLES

The following examples are presented only to illustrate certain specificembodiments of the invention, and should not be construed to limit thescope of the invention as set forth in the claims. There are manypossible other variations, as those of ordinary skill in the art willrecognize, within the spirit of the invention.

Example I

Catalyst samples are prepared.

Catalyst A: Steamed and calcined aluminum-phosphate-bound MFI zeolitespheres are prepared using the method of Example I in U.S. Pat. No.6,143,941. The pellets are impregnated with an aqueous solution ofammonium heptamolybdate, dried and calcined in dry air for 2 hours at538° C. to give 1.0 mass-% Mo on the catalyst.

Catalyst B: Steamed and calcined aluminum-phosphate-bound MFI zeolitespheres are prepared using the method of Example I in U.S. Pat. No.6,143,941. The pellets are impregnated with 1 mass-% Mo from an aqueoussolution of ammonium heptamolybdate, dried and calcined in dry air for 2hours at 538° C. The catalyst is contacted with 10 mass-% H₂S in H₂ for6 hours at 415° C. to give a sulfur level of 0.7 mass-% on the catalyst.

Catalyst C: Steamed and calcined aluminum-phosphate-bound MFI zeolitespheres are prepared using the method of Example I in U.S. Pat. No.6,143,941. The pellets are impregnated with 1 mass-% Mo from an aqueoussolution of ammonium heptamolybdate, dried and calcined in dry air for 2hours at 538° C. The catalyst is contacted with H₂S at ambienttemperature to give a sulfur level of 0.7 mass-% on the catalyst.

Catalyst D: Steamed and calcined aluminum-phosphate-bound MFI zeolitespheres are prepared using the method of Example I in U.S. Pat. No.6,143,941. The pellets are impregnated with an aqueous solution oftetra-ammine platinum chloride to give 0.038 mass-% platinum afterdrying and calcination at 538° C.

Example II

Catalysts A, B, C and D are evaluated in a pilot plant for theisomerization of a feed stream containing 7 mass-% ethylbenzene, 1mass-% para-xylene, 22 mass-% ortho-xylene and 70 mol-% meta-xylene. Thepilot plant runs are at a hydrogen to hydrocarbon ratio of 4:1, totalpressure of 1200 kPa, and weight hourly space velocity of 10 based onthe total amount of catalyst loaded. The pilot plant runs are summarizedin Table 1. The product data are taken at approximately 50 hours ofoperation.

TABLE 1 Catalyst A D (comparative) B C (comparative) EB Conversion, % 7575 75 75 WABT, ° C. 399 388 390 404 Para-xylene/xylene 23.3 23.7 23.623.7 Xylene loss, % 2.7 2.1 2.5 3.1 Toluene + 1.8 1.9 2.5 2.1Trimethylbenzene, mass-% yield C₆ Naphthenes, 0.015 0.005 0.002 0.080mass-% yield

Example III

During a pilot plant evaluation using feed and process conditions givenin Example II, Catalyst A is held at 404° C. and is contacted with feedwhich contains 1 mass-ppm sulfur added as thiophene for sufficient timeto pass 2 moles sulfur per mole molybdenum. After this time, thecatalyst performance is re-evaluated with feed without added sulfur. Theresults are shown in Table 2.

TABLE 2 Prior to After in-situ sulfiding sulfiding: EB Conversion, % 7575 WABT, ° C. 399 399 Para-xylene/xylene 23.3 23.5 Xylene loss, % 2.72.0 Toluene + Trimethylbenzene, mass-% yield 1.8 1.8 C₆ Naphthenes,mass-% yield 0.015 0.002

1. A catalyst comprising from 2 to 90 mass-% of molecular sieve having apore diameter of from about 4 to 8 angstroms and a molybdenumhydrogenation component present in an amount from about 0.1 to about 5mass-% molybdenum (elemental basis) which has been subjected tosufficient sulfiding so that the catalyst, under Evaluation Conditions,exhibits an ethylbenzene conversion of at least 75 mass-% and apara-xylene to total xylene mass ratio of at least about 0.233:1.
 2. Thecatalyst of claim 1 wherein the molecular sieve is a pentasil molecularsieve.
 3. The catalyst of claim 2 wherein the molecular sieve isMFI-type molecular sieve.
 4. The catalyst of claim 1 wherein thecatalyst comprises sulfur in a S:Mo atomic ratio of up to about 3:1. 5.The catalyst of claim 4 wherein the catalyst comprises sulfur in a S:Moatomic ratio between about 0.1 to 2:1.